The production of ultra-bright electron bunches using ionization injection triggered by two transversely colliding laser pulses inside a beam-driven plasma wake is examined via three-dimensional (3D) particle-in-cell (PIC) simulations. The relatively low intensity lasers are polarized along the wake axis and overlap with the wake for a very short time. The result is that the residual momentum of the ionized electrons in the transverse plane of the wake is much reduced and the injection is localized along the propagation axis of the wake. This minimizes both the initial thermal emittance and the emittance growth due to transverse phase mixing. 3D PIC simulations show that ultra-short (∼8 fs) high-current (0.4 kA) electron bunches with a normalized emittance of 8.5 and 6 nm in the two planes respectively and a brightness greater than 1.7 × 10 19 A · rad −2 · m −2 can be obtained for realistic parameters. The demonstration of the Linac Coherent Light Source (LCLS) as an X-ray free electron laser (X-FEL) [1] has given impetus to research on the fifth-generation light sources [2]. The goal is to make X-FELs smaller and cheaper while decreasing their wavelength and increasing their coherence and intensity. The FEL performance is partially determined by the brightness of the electron beam that traverses the undulator. The brightness is defined as B n = 2I/ǫ 2 n where I is the beam current and ǫ n is the normalized emittance of the beam. In order to make the length of the undulator needed to drive the SASE-FEL [3] into saturation, shorter, high current (∼kA), multi GeV electron beams with ǫ n ∼ 10nm will be needed. These emittances are an order of magnitude smaller than those from state-of-the-art photoinjector RF guns [4]. In this letter, we show the generation of ultrabright electron bunches using ionization injection triggered by two transversely overlapping laser pulses inside a beam-driven wake in plasma. In our scheme, the relatively low intensity lasers are polarized along the wake axis and overlap with the wake for a very short time. Particle-in-cell (PIC) simulations using OSIRIS [5] show that this geometry reduces the residual momentum of the ionized electrons in the transverse plane and localizes them along the propagation axis of the wake leading to an electron beam with a brightness greater than 10 19

Phase space matching between two plasma-accelerator (PA) stages and between a PA and a traditional accelerator component is a critical issue for emittance preservation of beams accelerated by PAs. The drastic differences of the transverse focusing strengths as the beam propagates between different stages and components may lead to a catastrophic emittance growth in the presence of both finite energy spread and lack of proper matching. We propose using the linear focusing forces from nonlinear wakes in longitudinally tailored plasma density profiles to provide exact phase space matching to properly transport the electron beam through two such stages with negligible emittance growth. Theoretical analysis and particle-in-cell simulations show how these structures may work in four different scenarios. Good agreement between theory and simulation is obtained.

The evolution of beam phase space in ionization-induced injection into plasma wakefields is studied using theory and particle-in-cell (PIC) simulations. The injection process causes special longitudinal and transverse phase mixing leading initially to a rapid emittance growth followed by oscillation, decay, and eventual slow growth to saturation. An analytic theory for this evolution is presented that includes the effects of injection distance (time), acceleration distance, wakefield structure, and nonlinear space charge forces. Formulas for the emittance in the low and high space charge regimes are presented. The theory is verified through PIC simulations and a good agreement is obtained. This work shows how ultra-low emittance beams can be produced using ionization-induced injection.The field of plasma based acceleration has experienced significant progress in the past decade [1]. GeV energy gain in centimeter-scale laser driven wakes (LWFA) has been achieved in many recent experiments [2][3][4][5]. In beam driven wakes (PWFA), high gradient acceleration has been sustained over meter-scale distances leading to more than 40GeV energy gain [6][7][8]. For future applications of wakefield accelerators such as FELs and colliders, the quality of the self-injected beams in plasma waves, namely the transverse and longitudinal emittances, need to be improved and controlled. Among the many injection schemes [9,10], ionization-induced injection methods have attracted significant interests due to its simplest and flexibility [5,[11][12][13][14][15][16]. However, the injection process involves complex phase space dynamics, and the achievable final beam quality strongly depends on this evolution process. This area of research is of fundamental importance for achieving beam quality well beyond what is achievable with current technology.In this letter, we examine carefully the effects that affect the beam phase space evolution in ionization-induced injection using a combination of theory and simulations. We found the evolution typically has three stages, and each stage can impact the final beam quality. In typical cases where the injection time is limited to few inverse plasma periods (2πω −1 p ) and the charge is low, the three stages are as follows. First, when ionization is occurring, the emittance of the injected beam grows quickly in time from the initial thermal emittance. Second, immediately following ionization, the emittance slowly decreases to a minimum value. Finally, the emittance again gradually increases to saturated values. If the ionization time is more than ∼ πω −1 p then the emittance grows to the saturated level during the first stage including an oscillatory behavior before it slowly decreases. In the "high" charge limit the emittance evolves monotonically towards the same saturated value.The theory reveals that the evolution in emittance described above is due to special longitudinal and transverse phase mixing of electrons born at different times.The derived expressions clearly show how the emittance dep...

Ionization injection triggered by short wavelength laser pulses inside a nonlinear wakefield driven by a longer wavelength laser is examined via multidimensional particle-in-cell simulations. We find that very bright electron beams can be generated through this two-color scheme in either collinear propagating or transverse colliding geometry. For a fixed laser intensity I, lasers with longer/shorter wavelength λ have larger/smaller ponderomotive potential (∝ Iλ 2 ). The two-color scheme utilizes this property to separate the injection process from the wakefield excitation process. Very strong wakes can be generated at relatively low laser intensities by using a longer wavelength laser driver (e.g., a 10 μm CO 2 laser) due to its very large ponderomotive potential. On the other hand, a short wavelength laser can produce electrons with very small residual momenta (p ⊥ ∼ a 0 ∼ ffiffi I p λ) inside the wake, leading to electron beams with very small normalized emittances (tens of nm). Using particle-in-cell simulations we show that a ∼10 fs electron beam with ∼4 pC of charge and a normalized emittance of ∼50 nm can be generated by combining a 10 μm driving laser with a 400 nm injection laser, which is an improvement of more than 1 order of magnitude compared to the typical results obtained when a single wavelength laser is used for both the wake formation and ionization injection. With the transverse colliding geometry, simulations show that similarly low emittance and much lower slice energy spread (∼30 keV, comparing with the typical value of few MeV in the longitudinal injection scheme) can be simultaneously obtained for electron beams with a few pC charge. Such low slice energy spread may have significant advantages in applications relevant to future coherent light sources driven by plasma accelerators.

Relativistic wakes produced by intense laser or particle beams propagating through plasmas are being considered as accelerators 1, 2 for next generation of colliders and coherent light sources 3 . Such wakes have been shown to accelerate electrons and positrons to several gigaelectronvolts (GeV) 4-10 , with a few percent energy spread 8-10 and a high wake-to-beam energy transfer efficiency 7 . However, complete mapping of electric field structure of the wakes has proven elusive. Here we show that a high-energy electron bunch can be used to probe the fields of such light-speed wakes with femtosecond resolution. The highly transient, microscopic wakefield is reconstructed from the density modulated ultra-short probe bunch

A new method capable of capturing coherent electric field structures propagating at nearly the speed of light in plasma with a time resolution as small as a few femtoseconds is proposed. This method uses a few femtoseconds long relativistic electron bunch to probe the wake produced in a plasma by an intense laser pulse or an ultra-short relativistic charged particle beam. As the probe bunch traverses the wake, its momentum is modulated by the electric field of the wake, leading to a density variation of the probe after free-space propagation. This variation of probe density produces a snapshot of the wake that can directly give many useful information of the wake structure and its evolution. Furthermore, this snapshot allows detailed mapping of the longitudinal and transverse components of the wakefield. We develop a theoretical model for field reconstruction and verify it using 3-dimensional particle-in-cell (PIC) simulations. This model can accurately reconstruct the wakefield structure in the linear regime, and it can also qualitatively map the major features of nonlinear wakes. The capturing of the injection in a nonlinear wake is demonstrated through 3D PIC simulations as an example of the application of this new method.

A new method for diagnosing the temporal characteristics of ultrashort electron bunches with linear energy chirp generated from a laser wakefield accelerator is described. When the ionizationinjected bunch interacts with the back of the drive laser, it is deflected and stretched along the direction of the electric field of the laser. Upon exiting the plasma, if the bunch goes through a narrow slit in front of the dipole magnet that disperses the electrons in the plane of the laser polarization, it can form a series of bunchlets that have different energies but are separated by half a laser wavelength. Since only the electrons that are undeflected by the laser go through the slit, the energy spectrum of the bunch is modulated. By analyzing the modulated energy spectrum, the shots where the bunch has a linear energy chirp can be recognized. Consequently, the energy chirp and beam current profile of those bunches can be reconstructed. This method is demonstrated through particle-in-cell simulations and experiment.

The transverse stability of the target is crucial for obtaining high quality ion beams using the laser radiation pressure acceleration (RPA) mechanism. In this letter, a theoretical model and supporting two-dimensional (2D) Particle-in-Cell (PIC) simulations are presented to clarify the physical mechanism of the transverse instability observed in the RPA process. It is shown that the density ripples of the target foil are mainly induced by the coupling between the transverse oscillating electrons and the quasi-static ions, a mechanism similar to the transverse two stream instability in the inertial confinement fusion (ICF) research. The predictions of the mode structure and the growth rates from the theory agree well with the results obtained from the PIC simulations in various regimes, indicating the model contains the essence of the underlying physics of the transverse break-up of the target. [6] and so on. Ideal one dimensional (1D) simulations show monoenergetic ion acceleration in the RPA process using a circularly polarized (CP) laser pulse[7-13] with high energy conversion efficiency. In reality, however, the finite transverse witdth of the laser pulse can deform the target shape, leading to electron heating and energy spectrum broading of the accelerated ions [10,11,14]. At the same time, 2/3D simulations also show that transverse density ripples can grow significantly, leading to some of the laser energy through and breaking up the target [10,11,[14][15][16][17][18][19]. This phenomenon shows up even for a laser pulse of infinite width and uniform intensity profile [11,19,20]. Various mechanisms have been proposed to explain the structrue of these ripples, such as Rayleigh-Taylor like (RT-like) instability [10,11,17,[19][20][21][22][23], Weibel like instability [16,18] and so on. However, these models have not been able to give accurate predictions of the mode structure and its growth rates for a wide range of laser and plasma parameters.In this letter, we show through theoretical analysis and PIC simulations that these surface ripples are more likely induced by the coupling between the transverse oscillating electrons and the quasi-static ions within the high density layer formed by the laser radiation pressure pushing the surface plasma forward in a process often called 'hole-boring" (H-B) [7,24]. As shown in Fig. 1(a), during this H-B process, soon after the laser impinges on the front surface of the target, a dynamic equilibrium between the laser pressure and the electrostatic field within the plasma is built, forming a quasi-static high density structure co-moving with the laser pulse [7]. Within this layer, the CP laser field oscillates at the laser frequency along both transverse directions albeit pi radians out of phase. A very small transverse ion density fluctuation can couple with the oscillating laser field to excite an electron oscillation. This oscillation in turn can couple with the oscillating laser field to generate a ponderomotive force with spatial variation, driving the electrons to...